1. Field of the Invention
This invention is directed to a solar collector system and in particular to an array of photovoltaic modules for an integrated solar power collector system.
2. Background and Related Art
Photovoltaic devices made of Selenium have been known since the 19.sup.th century. The silicon solar cell was the first known photovoltaic device that could convert a sufficient amount of sun's energy to power complex electronic circuits.
A photovoltaic cell is a solid-state device in which a junction is formed between adjacent layers of semiconductor materials doped with specific atoms, in order to create an n-type region and a p-type region. The photons incident on the semiconductor dislodge electrons from the valence band. These electrons, collected by the electric field at the junction, create a voltage which can be put at work in an external circuit.
A basic limit on the performance of these devices is that light photons lacking the energy needed to lift electrons from the valence to the conduction bands can not contribute to photovoltaic current, and also the energy transferred to electrons which exceeds the minimum excitation threshold, can not be recovered as useful electric current. Most of the photon energy not recovered as electricity, is converted to thermal energy in the cell. The overall effect is that the efficiency of solar cells does not exceed 16% in commercial devices.
As the energy collected is proportional with the surface exposed to solar radiation, and also in order to accommodate the electrical characteristics of the solar cells, in most application 36 (4.times.9) photovoltaic cells are grouped in a module, the cells being connected in series. The wires between the cells are arranged inside the module and not accessible. The only accessible connections are the "+" terminal of the first cell in the series and the "-" terminal of the last cell.
A module usually has an area of 1.5'.times.3' or 4'.times.2'. Larger modules with an area of 16 square feet are also commercially available. A cell generates approximately 0.5 volts, and as a result, a module generates approximately 20 volts open circuit, giving a short-circuit current of 4 to 6 amperes. Still, the power generated by a module is rather low, a fill factor of only 0.5-0.8 can be obtained, depending on the quality of the cells.
Therefore, many applications use arrays of modules which are connected to each other in accordance with current and voltage requirements of the application. There are also various ways for electrically connecting the modules; in general, the positive and negative poles of all modules are wired to two bus-bars, or are wired with conductors placed in channels specifically provided on the back of the modules.
It is known practice to mount solar modules on rooftops where they are most likely to receive a maximum amount of sunlight without interference from trees or nearby constructions. One method for mounting an array of solar modules on a roof is to first assemble the modules on a base, to form the array, and then to secure the array on the existing rooftop. Special consideration should be given to the wind load, the weight of the snow, and the extra weight of the modules, when designing such a roof-over roof assembly.
Another method for mounting an array on the roof is to mount the solar modules individually directly on the roof. In this case, there are a number of operations which must be performed on the roof, such as installing the modules, interlocking the modules to each other, wiring the modules and treating the surface thus obtained to obtain a weather--resistant roof. The modules are fragile and may break if stepped on. As such, walkways must be provided on the roof, which reduce the surface dedicated to the modules.
A photovoltaic roofing assembly is disclosed in U.S. Pat. No. 5,505,788 (issued to Dinwoodie on Apr. 9, 1996), where the modules are disposed as a layer on top of the roof, and separated therefrom by pre-formed spacers, pedestals or supports. Use of spacers enables heat exchange with a convecting fluid flowing between the modules and the roof. Preferably, the cooling fluid is air or other gaseous fluid, or could also be a liquid. Besides the disadvantages listed above, it is difficult to mount the spacers on the roof and then the modules on the spacers as in the system disclosed by this patent.
There is a need to provide a method for installing an array of photovoltaic modules directly on the roof, which is simple, cost-effective and easy to install.